1. Field of the Invention
The invention relates to ball mills. Specifically, the invention relates to ball mills for continuously pulverizing materials including microorganisms and dispersion solids.
2. Description of the Background Art
Ball mills of the subject type are known in various embodiments. The term "ball" refers not only to the preferred true spherical grinding elements but also to any other grinding elements which are suitable for producing pulverization of solid particles of a material undergoing grinding, by means of mutual compressive rolling, and compressive rolling at the boundary surfaces of the milling or pulverization space.
As a rule, the spheres used are finely ground or of small diameter and are comprised of hard wear-resistant steel, hard metal, glass, or ceramic. However, grinding elements comprised of other materials can be employed. In the past, grains of sand were used as a primary or secondary grinding element. Often the only way these could be converted to a form suitable for use as an aid in compressive rolling was by a pregrinding process.
Swiss Pat. No. 639,567 discloses a gap-type ball mill intended for continuous operation, wherein an impeller member of the rotor, having a wedge-shaped cross section, surrounds the mill axis at a radial distance therefrom, and fits operatively into a pulverization space of like shape in the stator. The material being pulverized flows over the entire impeller which is mounted on a rotor disc in an extended acceleration phase, and around the tip of the wedge, and then is passed inwardly to the extend of about half the radius, to an exit structure.
The grinding elements flow generally in the same path as the material being pulverized, but they are separated out by a separating device before they exit and are passed through a ball return channel running outward at an angle in the rotor disc. Moving thus outward by centrifugal force, they are returned to the region where the material being pulverized is admitted or the "inlet region". From there they are recirculated through their closed circulation path.
In this way, high energy-densities are concentrated onto a small surface and in a small space, thereby enabling high pulverizing output in a compact mill, combined with low manufacturing cost. However, it is a challenge to provide a configuration of the separating device and ball return system which does not lead to jamming of the balls. The consequences of jamming are aggravated by the fact that the cooperating parts such as the rotor and stator all have unitary structures which can only be repaired and replaced as a unit.
There are other known embodiments, particularly in connection with the ring-type gap mills, wherein parts active in the pulverization are replaceable. Such an example is German Pat. No. 3,526,724. However, particularly with high speed mills, this leads to problems in the repair process with regard to the mounting means for the replaceable mill elements. On the other hand, the principle is promising in that it enables the combination of successive milling stages or even disparate milling processes. Another advantage is that rotor and stator discs with axial passages or openings through them are provided. Nonetheless, at high material throughputs, all the grinding elements being circulated must be intercepted by a single interstage separating device, which can readily lead to jamming of the large number of said grinding elements, thereby disabling the mill.
The present invention is a gap-type ball mill as described above. The underlying problem to be solved by the invention is to refine the gap-type ball mill in the simplest manner possible, particularly for purposes of breaking down microorganisms, such that the mill can be used for a variety of tasks without the danger and disadvantage of jamming of the balls, and can be easily maintained and repaired, so that downtime is low.